Pump

ABSTRACT

Provided is a pump and a method of controlling a pump. The pump and method are particularly for use in dispensing reagents, for example in flow chemistry, and more particularly on a laboratory scale. The pump aims to provide a substantially constant output flow of fluid. The pump comprises: a motor; a peristaltic pump having a rotor driven by the motor; a pressure sensor monitoring the pressure of the pumped fluid downstream of the pump; and a control unit which controls the motor by adjusting the standard operating speed of the motor according to the pressure detected by the pressure sensor, such that the pump operates continuously at a rate set by an operator. Embodiments of the pump make use of lookup tables to determine a desired position of the pump rotor at each point in the cycle with the entry point into the lookup table being determined by the feedback from the pressure sensor. Long term changes in the performance of the pump can also be accounted for by changing the entry in the lookup table which is consulted.

FIELD OF THE INVENTION

The present invention relates to a pump and a method of controlling apump. The invention is particularly, but not exclusively, concerned witha pump for use in flow chemistry or for dispensing reagents and a methodof controlling such a pump.

BACKGROUND OF THE INVENTION

Flow chemistry is a process whereby a chemical reaction is run in acontinuously flowing stream rather than in one or more batches. Althoughlong used in manufacturing processes on a large scale, flow chemistryhas only recently been developed in the research/laboratory environment.

A key characteristic of flow chemistry is the need to pump liquids toand normally through reaction vessels. However, this pumping can be thesource of problems, particularly on the laboratory scale, for examplewhere corrosive reagents or reagents with a high concentration ofparticulates are used in the reaction.

Where reagents contain particulates existing pumps used in laboratoryscale flow chemistry tend to be peristaltic pumps operating at around1-2 bar of output pressure. These pumps have typically been unable todeal with corrosive reagents, have operated at relatively low pressureand have had irregular output flow rate.

Recent advances in materials have led to the development of compositetubes containing perfluoroelastomers and fluoroelastomers. Examples ofthese materials include the style 400 and style 500 tubes supplied byGORE® Reinforced Elastomers W.L. Gore & Associates, Inc. 2401 SingerlyRoad Elkton, Md. 21921. By selecting the correct material for thereagent being pumped the tubes are able to cope with acids, bases andorganic solvents without corrosion or physical deterioration, but thisdoes not address the problems of low pressure and irregular flows.

Various forms of pumps have been developed which address the aboveproblems. Syringe pumps are used, but these are obviously limited intheir capacity, even if paired, which requires change over in order toprovide a continuous supply of reagent. Positive displacement pumps havealso been used which have small pistons (of the order of 541) whichconstantly draw in reagent from a supply and pump it out to the reactionchamber. However, the pistons in such pumps require non-return valves toprevent the reagent from being forced back into the supply rather thanout to the reaction chamber and such valves do not cope well withprecipitates, air bubbles in the reagent or immiscible liquids.

FIG. 6 shows the performance of a typical prior art peristaltic pump,which is controlled to achieve a constant speed of rotation (middlegraph). The top and bottom graphs illustrate that, in thisconfiguration, there is a wide variation in pressure and flow ratedownstream of the pump.

Aspects of the present invention seek to address, overcome or amelioratethe above shortcomings of existing pumps used in flow chemistry.Preferably such pumps are able to cope with a wide range of reagents,including those with high levels of precipitates, and/or able tomaintain a constant or substantially constant flow rate and/or able toprovide reagents at a high pressure (e.g. up to 10 bar or more) to areaction chamber.

SUMMARY OF THE INVENTION

At its broadest, a first aspect of the present invention provides a pumpwhich uses feedback from a pressure sensor to control the operation ofthe pump. The pump is preferably suitable for use in dispensingreagents, and particularly, but not exclusively, in flow chemistryapplications.

Accordingly, a first aspect of the present invention preferably providesa pump for use in dispensing reagents, the pump comprising: a motor; aperistaltic pump having a rotor driven by the motor; a pressure sensormonitoring the pressure of the pumped fluid downstream of the pump; anda control unit which controls the motor by adjusting the standardoperating speed of the motor according to the pressure detected by thepressure sensor, such that the pump operates continuously at a rate setby an operator.

Adjusting the operating speed according the detected pressure provides amore accurate and more rapid feedback than feedback based on flow ratesand eliminates the need for mass flow sensing which can be highlyproblematic with corrosive solutions.

The output flow of the pump has been found to be very closely correlatedto output pressure, even though it is pumping against a back pressureregulator which attempts to regulate pressure. Pressure regulators areslightly non-linear, so that an increase in flow produces a smallincrease in pressure. Other effects, such as system tubing losses, andinertial mass of the fluid being pumped, combine to the effect thatvariations in outlet flow produce detectable variations in outletpressure.

Therefore, precisely regulating outlet pressure, by means of adjustingrotor speed, will produce a steady outlet flow. It has been found thatpumps according to embodiments of the present invention are able tomaintain outlet flow downstream of the pump within a 5% variation of thedesired flow rate.

Preferably the control unit compares the pressure detected by thepressure sensor with a target pressure and adjusts the speed of themotor accordingly.

Preferably the sole user input to the controller is a speed set point.However, in other arrangements the user input may be a desired flowrate.

Preferably the control unit also receives a measurement of the currentposition of the rotor and compares that position to a desired positionof the rotor, and adjusts the speed of the motor to compensate for anydifference between the current position and the desired position.

In this configuration, there are two feedback parameters available theactual rotor position and the fluid pressure at the pump outlet.

Preferably the control unit determines a desired position of the rotorthroughout its rotary cycle from a lookup table according to the rateset by the operator.

The transfer function of a pump is typically highly non-linear, andvaries over operating pressure, and with tubing type and condition. Tocompensate for this, the lookup table can provide curves describing theinverse of the transfer function which can be captured using a purelyopen-loop controller, running at constant pressure and flow.

Ideally several such transfer function maps are captured and provided atdifferent pressures. In this arrangement the desired position of therotor throughout the rotary cycle in the lookup table entries can bedetermined by the position of the rotor required to maintain a constantoutput pressure.

Preferably the control unit adjusts or selects the point of entry intothe lookup table from which the desired position of the rotor isdetermined according to a target pressure.

The appropriate transfer function map can thus be selected for use, withthe target pressure largely dictating the map choice. This transferfunction map then forms an idealised model of where the rotor should beat any point in time, to produce steady flow, and preferably dominatesthe behaviour of the controller to provide a system largely immune tooutside disturbances.

Preferably, if the lookup table does not contain an entry for the outputpressure in question, the control unit interpolates between adjacententries in the lookup table to determine the desired position of therotor throughout the rotary cycle.

The entries in the lookup table are typically discrete entries for theselected pressures. These pressures may not exactly match the targetpressure. Accordingly, the lookup preferably interpolates between pairsof adjacent transfer functions, so effectively a continuous range ofcurves are generated for a continuous range of pressure.

Preferably the control unit receives a measurement of the currentposition of the rotor and compares that position to a desired positionof the rotor, and adjusts the speed of the motor to compensate for anydifference between the current position and the desired position.

In a particularly preferred arrangement the control unit calculates thespeed that will cause the difference between the current position andthe desired position to zero by the time of the next measurement.

In one embodiment, the motor is a stepper motor. A stepper motor isparticularly preferred as it provides constant information about theposition of the motor drive shaft and therefore the position of therotor. However, other motor types could be used, but preferably themotor chosen allows precise speed control and feedback information aboutthe position of the rotor either from the drive of the motor itself orfrom a position encoder.

Preferably the control unit monitors the performance of the pump overtime using the measured pressure and adjusts the entry in the lookuptable which is consulted according to the observed performance of thepump.

This allows the control unit to take account of variations in the tubingproperties in the pump, and over the life of the tube, and possiblybetween tubes of the same type due to manufacturing differences.

For example, the control unit may be arranged such that, if the measuredpressure undershoots a target pressure after the high speed section, ahigher pressure map is used with a longer fast section. Conversely, ifthe measured pressure overshoots after the fast section, a lowerpressure map is selected.

Preferably any change is blended in gradually over the course of arevolution of the pump.

Providing long term matching of the selected lookup table entry based onthe performance of the pump allows lower short term control inputs to bemade, which aids stability and immunity to fluctuations in flow causedby pressure transients from sources external to the pump, such asanother pump operating upstream or downstream of the pump in question.

Preferably a target pressure level is maintained to provide a set pointfor the control unit. In a preferred arrangement, the target pressure isa low-pass filtered version of the outlet pressure. The filter ispreferably designed to maintain a constant level during normaloperation, ignoring the pressure transients caused by the fast sectionof rotor travel. The filter may be gated so that it will track to theactual pressure level quickly during system start-up and pressureregulator adjustments. If the actual pressure maintains a levelsignificantly different from the set point pressure for too long(several hundred milliseconds) the target pressure is quickly adjustedto match.

Preferably the control unit is arranged to adjust the value used to lookup in the lookup table, not the output value of the lookup table.Typically for a peristaltic pump, the transfer function has a period ofslow movement (corresponding to the standard rotation of the rotor withthe roller(s) in contact with the tubing) and a short period of fastmovement (corresponding to the point at which a roller disengages withthe tubing and so the rotor has to accelerate to maintain the pressurein the tubing and avoid flow back into the section of tubing behind thedisengaged roller.

By adjusting the input to the lookup table based on the measuredpressure, the effect of the control unit feedback can be limited to asmall range of travel on the input axis, which produces an even smallereffect on the rotor movement during the slow portions of the transferfunction. However, there is some variability in the timing of therelease of the nip and the above described arrangement can allow thecontrol unit to execute the full magnitude of the fast section of travelearly, or delay it slightly, depending on the pressure sensor feedback.

The fast section of the transfer function of a peristaltic pump canprovide for about 50% of the total movement of the rotor in a singlecycle at higher pressures, so the control unit would have to have limitsof at least this range if it were adjusting map output values. Using theinput side of the lookup table, adjustments of only around 5-10% of theinput value may be needed.

Furthermore, the limited influence of the feedback from the pressuresensor in the slow portions of the lookup table can provide significantimmunity of the pump control to external pressure disturbances, such asthose caused by another pump operating further downstream.

The pump of the first aspect may have some, all or none of the abovedescribed optional and preferred features.

At its broadest, a second aspect of the present invention provides amethod for controlling a pump which uses the pressure detecteddownstream of the pump to control the operation of the pump. The pumpbeing controlled is preferably suitable for use in dispensing reagents,and particularly, but not exclusively, in flow chemistry applications.

Accordingly, a second aspect of the present invention preferablyprovides a method of controlling a peristaltic pump having a rotordriven by a motor, the method comprising the steps of: receiving adesired operating rate from an operator; detecting the pressure of thepumped fluid downstream of the pump: and adjusting the standardoperating speed of the motor according to the detected pressure suchthat the pump operates continuously at the desired operating rate.

Adjusting the operating speed according the detected pressure provides amore accurate and more rapid feedback than feedback based on flow ratesand eliminates the need for mass flow sensing which can be highlyproblematic with corrosive solutions.

The output flow of the pump has been found to be very closely correlatedto output pressure, even though it is pumping against a back pressureregulator which attempts to regulate pressure. Pressure regulators areslightly non-linear, so that an increase in flow produces a smallincrease in pressure. Other effects, such as system tubing losses, andinertial mass of the fluid being pumped, combine to the effect thatvariations in outlet flow produce detectable variations in outletpressure.

Therefore, precisely regulating outlet pressure, by means of adjustingrotor speed, will produce a steady outlet flow. It has been found thatmethods according to embodiments of the present invention are able tomaintain outlet flow downstream of the pump within a 5% variation of thedesired flow rate.

Preferably the method further includes the steps of: comparing thepressure detected by the pressure sensor with a target pressure; andadjusting the speed of the motor accordingly.

Preferably the sole user input to the controller is a speed set point.However, in other arrangements the user input may be a desired flowrate.

The method preferably further includes the steps of: measuring thecurrent position of the rotor; comparing that position to a desiredposition of the rotor, and adjusting the speed of the motor tocompensate for any difference between the current position and thedesired position.

In this configuration, there are two feedback parameters available theactual rotor position and the fluid pressure at the pump outlet.

Preferably the method further includes the step of determining a desiredposition of the rotor throughout its rotary cycle from a lookup tableaccording to the rate set by the operator.

Preferably the desired position of the rotor throughout the rotary cyclein the lookup table entries is determined by the position of the rotorrequired to maintain a constant output pressure.

The transfer function of a pump is typically highly non-linear, andvaries over operating pressure, and with tubing type and condition. Tocompensate for this, the lookup table can provide curves describing theinverse of the transfer function which can be captured using a purelyopen-loop controller, running at constant pressure and flow.

Ideally several such transfer function maps are captured and provided atdifferent pressures. In this arrangement the desired position of therotor throughout the rotary cycle in the lookup table entries can bedetermined by the position of the rotor required to maintain a constantoutput pressure.

The method preferably further includes the step of adjusting the pointof entry into the lookup table from which the desired position of therotor is determined according to a target pressure.

The appropriate transfer function map can thus be selected for use, withthe target pressure largely dictating the map choice. This transferfunction map then forms an idealised model of where the rotor should beat any point in time, to produce steady flow, and preferably dominatesthe behaviour of the control method to provide a system largely immuneto outside disturbances.

The entries in the lookup table are typically discrete entries for theselected pressures. These pressures may not exactly match the targetpressure. Accordingly, the lookup preferably interpolates between pairsof adjacent transfer functions, so effectively a continuous range ofcurves are generated for a continuous range of pressure.

Accordingly, the method preferably further includes the step of, if thelookup table does not contain an entry for the output pressure inquestion, interpolating between adjacent entries in the lookup table todetermine the desired position of the rotor throughout the rotary cycle

In a particularly preferred arrangement the method calculates the speedthat will cause the difference between the current position and thedesired position to zero by the time of the next measurement.

In one embodiment, the motor is a stepper motor. A stepper motor isparticularly preferred as it can provide constant information about theposition of the motor drive shaft and therefore the position of therotor. However, other motor types could be used, but preferably themotor chosen allows precise speed control and feedback information aboutthe position of the rotor either from the drive of the motor itself orfrom a position encoder.

Preferably the method further includes the steps of: monitoring theperformance of the pump over time using the measured pressure; andadjusting the entry into the lookup table which is consulted accordingto the observed performance of the pump.

This allows the method to take account of variations in the tubingproperties in the pump, and over the life of the tube, and possiblybetween tubes of the same type due to manufacturing differences.

For example, the method may operate such that, if the measured pressureundershoots a target pressure after the high speed section, a higherpressure map is used with a longer fast section. Conversely, if themeasured pressure overshoots after the fast section, a lower pressuremap is selected.

Preferably any change is blended in gradually over the course of arevolution of the pump.

Providing long term matching of the selected lookup table entry based onthe performance of the pump allows lower short term control inputs to bemade, which aids stability and immunity to fluctuations in flow causedby pressure transients from sources external to the pump, such asanother pump operating upstream or downstream of the pump in question.

Preferably a target pressure level is maintained to provide a set pointfor the control method. In a preferred arrangement, the target pressureis a low-pass filtered version of the outlet pressure. The filter ispreferably designed to maintain a constant level during normaloperation, ignoring the pressure transients caused by the fast sectionof rotor travel. The filter may be gated so that it will track to theactual pressure level quickly during system start-up and pressureregulator adjustments. If the actual pressure maintains a levelsignificantly different from the set point pressure for too long(several hundred milliseconds) the target pressure is quickly adjustedto match.

Preferably the method works by adjusting the value used to look up inthe lookup table, not the output value of the lookup table. Typicallyfor a peristaltic pump, the transfer function has a period of slowmovement (corresponding to the standard rotation of the rotor with theroller(s) in contact with the tubing) and a short period of fastmovement (corresponding to the point at which a roller disengages withthe tubing and so the rotor has to accelerate to maintain the pressurein the tubing and avoid flow back into the section of tubing behind thedisengaged roller.

By adjusting the input to the lookup table based on the measuredpressure, the effect of the control feedback can be limited to a smallrange of travel on the input axis, which produces an even smaller effecton the rotor movement during the slow portions of the transfer function.However, there is some variability in the timing of the release of thenip and the above described arrangement can allow the control method toexecute the full magnitude of the fast section of travel early, or delayit slightly, depending on the pressure sensor feedback.

The fast section of the transfer function of a peristaltic pump canprovide for about 50% of the total movement of the rotor in a singlecycle at higher pressures, so the control unit would have to have limitsof at least this range if it were adjusting map output values. Using theinput side of the lookup table, adjustments of only around 5-10% of theinput value may be needed.

Furthermore, the limited influence of the feedback from the pressuresensor in the slow portions of the lookup table can provide significantimmunity of the pump control to external pressure disturbances, such asthose caused by another pump operating further downstream.

The method of the second aspect may have some, all or none of the abovedescribed optional and preferred features.

The method of this aspect is preferably used in conjunction with a pumpaccording to the above first aspect, including some, all or none of theoptional or preferred features of that aspect. However, the method maybe used in conjunction with alternative pumps.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a perspective view of a pump according to an embodiment ofthe present invention;

FIG. 2 shows a detailed view of the pump of FIG. 1 with the casingremoved;

FIG. 3 shows a sectional view of the pump of FIG. 1;

FIG. 4 illustrates, in schematic form, a method of controlling a pumpaccording to an embodiment of the present invention;

FIG. 5 is a graph showing the desired rotor position against time for arange of different operating pressures of a pump;

FIG. 6 is a graph showing the operation of a prior art peristaltic pumpand has already been described; and

FIG. 7 is a graph showing the operation of a pump according to anembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of a peristaltic pump 10 according to anembodiment of the present invention. The pump has a stepper motor 20, apump unit 30 and circuit board 40. Further control circuitry andconnections, discussed in more detail below, connect the circuit board40 and the stepper motor 20 and control the operation of the pump 10.

The pump unit 30 is arranged to pump reagents from inlet 34 out throughoutlet 32. The inlet 34 is generally connected, in use, to a source of areagent, such as a storage vat or bottle, or to the output of an earlierreaction system. The outlet 32 is generally connected, in use, to areaction chamber for conducting flow chemistry. Suitable sources ofreagents and reaction chambers are well known in the art and will not bedescribed further here.

FIG. 2 shows the detail of the pump unit 30 with the front coverremoved. The pump unit consists of a standard, albeit high quality andrugged, peristaltic pump configuration in which a flexible elastomerictube 33 which provides fluid communication between the inlet 34 and theoutlet 32 is sandwiched in a U-shaped configuration between the fixedblock 37 of the pump unit and a rotor 35. Mounted on the rotor are threerollers (also commonly referred to as “shoes” or “wipers”) 36 a, b & c.When the pump unit 30 is driven by the stepper motor 20, the rotor 35rotates (in a clockwise direction as viewed in FIG. 2) causing therollers to engage with the tube 33 to create a “nip” which is then movedaround the tubing by the motion of the roller in the known manner,causing fluid to be transferred from the inlet 34 to the outlet 32 andpressure to be applied to the fluid in the subsequent tubing downstreamfrom the outlet 32 in the known manner.

Where the pump is to be used with strong acids, a fluoroelastomer tubing33 is preferred. Where the pump is to be used with organic solvents, aperfluoroelastomer tubing 33 is preferred.

Although the pump unit 30 shown in FIG. 2 has three rollers, any numberof rollers may be used. In particular, there may be six rollers mountedon the rotor. The selection of the number of rollers will depend on theintended use of the pump. As is well known, a greater number of rollersgenerally gives a smoother flow and can allow increased pumpingpressure. However, a larger number of rollers generally necessitates alarger and therefore more expensive pump.

FIG. 3 shows a cross-section of the pump 10 as viewed from the side. Thedrive shaft 21 of the stepper motor 20 is connected to the drive shaft23 of the pump unit 30 by a rigid coupling 22. The provision of a rigidcoupling between the motor and the shaft avoids oscillation of the pumpdrive shaft (and therefore the rotor 35 and rollers 36), particularlywhen the pump is operating at high pressures and therefore there isconsiderable change in the reactionary torque on the pump drive shaft 23when a roller 36 engages or disengages the tube.

An optocoupler 25 is mounted on the coupling 22 to provide a shaftposition reading once every revolution to the control unit (not shown).The optocoupler comprises a optical sensor 25 a through which a slotteddisc 25 b which is rigidly mounted to the coupling 22 passes. Thisprovides an input to the motor controller as described in more detailbelow.

Downstream of the outlet 32, a pressure sensor 38 is mounted whichdetects the pressure in the tubing and feeds this back to the controlunit so that the motor speed can be adjusted accordingly, as describedbelow. The pressure sensor is a strain gauge pressure transducer withwetted parts manufactured from alumina ceramic as supplied by RoxspurMeasurement and Control Ltd 1-4 Campbell Court, Bramley, Tadley,Hampshire, RG26 5EG, UK.

The peristaltic pump unit 30, when pumping against a back pressure, hasa highly nonlinear transfer function. With the motor running at constantspeed, output flow is in the forwards direction until a roller 36releases a nip in the tubing, which causes the pressurised fluiddownstream of the pump to rush backwards into the now unpressurisedsection of the tubing, resulting in momentary reverse flow and, in mostcases, a loss in output pressure (depending on the volume and complianceof the system downstream of the pump.)

Accordingly, the pump of an embodiment of the present invention has acontrol system which controls the stepper motor based on the detectedpressure. The method of controlling the pump set out below constitutes afurther embodiment of the present invention. FIG. 4 shows a schematicoverview of the control system and its operation.

Viewed as a whole, the objectives of the control system are twofold:

-   -   To increase the speed of the rotor as the nip releases, so that        a steady flow is produced.    -   To govern the mean speed of the rotor so that a predetermined        flow rate is maintained.

The output flow of the pump has been found to be very closely correlatedto output pressure, even though it is pumping against a back pressureregulator which attempts to regulate pressure. Pressure regulators areslightly non-linear, so that an increase in flow produces a smallincrease in pressure. Other effects, such as system tubing losses, andinertial mass of the fluid being pumped, combine to the effect thatvariations in outlet flow produce detectable variations in outletpressure.

Therefore, precisely regulating outlet pressure, by means of adjustingrotor speed, will produce a steady outlet flow.

If the outlet flow and pressure are steady, the effects of the systemdownstream of the pump (which may have variable volume and compliance)are negated.

The pump aims to deliver the correct flow rate in the presence of systempressure variations, including pressure fluctuations introduced by otherpumps in the system. This is achieved by closely following apredetermined operating curve, and only allowing the pump position todeviate slightly from this curve to correct for pressure errors, themain source being some variability in the precise moment the rollerreleases the tubing nip.

The sole user input to the controller is the speed set point. There aretwo feedback parameters available (as discussed above in relation to thepump): the actual rotor position, and the fluid pressure at the pumpoutlet.

The transfer function of the pump is highly non-linear, and varies overoperating pressure, and with tubing type and condition. To compensatefor this, curves describing the inverse of the transfer function arecaptured using a purely open-loop controller, running at constantpressure and flow. Ideally several transfer function maps 52 arecaptured at different pressures.

The appropriate map is selected for use by the Performance Monitor 55,with the current Target Pressure largely dictating the map choice. Thisforms an idealised model of where the rotor should be at any point intime, to produce steady flow, and dominates the behaviour of thecontroller to provide a system largely immune to outside disturbances.

The map lookup interpolates between pairs of adjacent maps, soeffectively a continuous range of curves are generated for a continuousrange of pressure.

An actual set of eight maps derived at different operating pressuresfrom a pump according to the embodiment described above is shown in FIG.5, with time on the X axis, and desired rotor position on the Y axis.The higher line on the left hand side of the point of inflexion of thegraphs (and the lower line on the right hand side of the point ofinflexion) is at zero pressure, whilst the lower line on the left handside of the point of inflexion of the graphs (and the higher line on theright hand side of the point of inflexion) is at maximum pressure. Asthe pump has 3 rollers, only ⅓ of the rotation is recorded and thisinformation is used 3 times for each complete revolution of the rotor.

The Stepper Motor Driver 53 controls the speed of the motor, andprovides position feedback. In this embodiment, a stepper motor is usedwith a 1:1 drive gearing to the rotor. Other motor types could be used,providing they allow precise speed control and the position of the rotoris available for feedback, either as a model in the driver or via aposition encoder.

The actual motor position is subtracted from the demand position derivedfrom the selected Transfer Function Map 52, to produce a position error.The speed controller calculates the speed that will bring the calculatederror to zero by the time of the next controller cycle. This allows thestepper motor to be micro-stepped smoothly using a 20 kHz signalgenerator, while using a much slower controller cycle, without thecontroller cycle frequency being audible on the stepper drive.

The Target Pressure signal largely dictates which transfer functionmap(s) are used. The Performance Monitor 55 adjusts the map selectionslightly based on the observed pump performance. The elastomer tubingproperties vary between tubing types, and over the life of the tube, andpossibly between tubes of the same type due to manufacturingdifferences.

If the pressure undershoots after the high speed section, a higherpressure map is used with a longer fast section. If the pressureovershoots after the fast section, a lower pressure map is selected.

Only small changes to the map selection are made, and the change isblended in gradually over the course of a revolution of the pump.

Matching the map choice to the tubing properties allows lowerproportional integral derivative (PID) controller gains to be used,which aids stability and immunity to fluctuations in flow caused bypressure transients from sources external to the pump, such as anotherpump operating upstream or downstream of the pump in question.

A target pressure level is maintained to provide a set point for the PIDController 57. The target pressure is a heavily low-pass filteredversion of the outlet pressure. The filter is designed to maintain aconstant level during normal operation, ignoring the pressure transientscaused by the fast section of rotor travel. The filter is gated so thatit will track to the actual pressure level quickly during systemstart-up and pressure regulator adjustments. If the actual pressuremaintains a level significantly different from the set point pressurefor too long (several hundred milliseconds) the target pressure isquickly adjusted to match.

The PID Controller 57 is arranged to match the outlet pressure to thetarget pressure. If the outlet pressure falls, pump speed is increased,and vice versa. The controller regularly experiences saturationconditions: the motor speed and acceleration are limited, as is themagnitude of the PID controller output. Under saturation, the integratoris disabled to prevent integral wind-up in the saturated direction.

The integrator also has a time-decay function that is dependent on therotor speed, to prevent it slowly winding up to saturation over time.

The pump is constrained to closely follow the transfer function map.This is achieved by limiting the PID controller output level, whichcorresponds to X-axis displacement in FIG. 5.

The index used to look up into the Transfer Function Map 52 is driven bythe set-point speed. The speed value is integrated with respect to timein Integrator 51 to calculate the open-loop index.

The output of the PID Controller 57 is used to modify the lookup indexderived from the Integrator 51 slightly, to allow the PID Controller 57a small influence over the rotor position. The PID level is added ontothe open-loop index to produce the actual index used for lookup.

The PID Controller 57 adjusts the value used to look up into the map,not the map output value. This has the advantage that the controller canbe limited to a small range of x-axis travel, which produces an evensmaller y-axis displacement during the linear portions of the map.However, there is some variability in the timing of the release of thenip, so this arrangement allows the controller to execute the fullmagnitude of the fast section of travel early, or delay it slightly,depending on the PID controller output.

The fast section (the near vertical chart area around the point ofinflexion in FIG. 5) occupies about 50% of the y-axis at higherpressures, so the PID controller would have to have limits of at leastthis range if it were working with map output values. Using the inputside of the map, a range of only around 5-10% of the map is needed.

The very limited influence of the PID Controller 57 in the linearsections of the map provides a large amount of immunity to externalpressure disturbances, such as those caused by another pump.

The pump speed can preferably be adjusted over a range of at least 100:1without changing the transfer function maps. At lower speeds, the mapcurves are too slow for effective operation if the controller werepurely open loop. The closed loop controller causes the lookup index toaccelerate as the pressure drops, so the fast section of travel isexecuted in the same time as when the pump is running at full speed, andlimited only to the motor acceleration and maximum speed settings.

The performance of the pump of the above embodiment, as operated by themethod described above, is illustrated in FIG. 7. It can be seen thatthe instantaneous speed of the rotor increases massively (middle graph)at the point at which each roller 36 disengages the tubing 33 in orderto accommodate the resulting decrease in downstream pressure (topgraph). The downstream pressure is therefore controlled so that it onlytransiently spikes downwards as each roller 36 disengages the tubing 33with the control unit operating to immediately rectify this decrease andreturn the operation of the pump to the target pressure. As a result ofthis, the flow rate downstream of the pump (bottom graph) is maintainedbroadly constant over the operation of the pump.

Using the pump or method of the above embodiments, it is possible tomaintain the flow rate of the fluid being pumped downstream of the pumpwithin 5% of the desired flow rate at all times during the steady stateoperation of the pump.

The method of the above embodiment may be implemented in a computersystem (in particular in computer hardware or in computer software) inaddition to the structural components and user interactions described.

The term “computer system” includes the hardware, software and datastorage devices for embodying a system or carrying out a methodaccording to the above described embodiments. For example, a computersystem may comprise a central processing unit (CPU), input means, outputmeans and data storage. Preferably the computer system has a monitor toprovide a visual output display (for example of the operation of thepump, or of various real time outputs such as the speed, pressure orflow rate). The data storage may comprise RAM, disk drives or othercomputer readable media. The computer system may include a plurality ofcomputing devices connected by a network and able to communicate witheach other over that network.

The method of the above embodiment may be provided as computer programsor as computer program products or computer readable media carrying acomputer program which is arranged, when run on a computer, to performthe method(s) described above.

The term “computer readable media” includes, without limitation, anymedium or media which can be read and accessed directly by a computer orcomputer system. The media can include, but are not limited to, magneticstorage media such as floppy discs, hard disc storage media and magnetictape; optical storage media such as optical discs or CD-ROMs; electricalstorage media such as memory, including RAM, ROM and flash memory; andhybrids and combinations of the above such as magnetic/optical storagemedia.

The method and pump described in the above embodiments are preferablycombined and used in conjunction with each other, but this is notnecessary and, in particular the method may be used to control a pumpwith an alternative configuration.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

What is claimed is:
 1. A pump for use in dispensing reagents, the pumpcomprising: a motor; a peristaltic pump having a rotor driven by themotor; a pressure sensor monitoring the pressure of the pumped fluiddownstream of the pump; and a control unit which controls the motor byadjusting the standard operating speed of the motor according to thepressure detected by the pressure sensor, such that the pump operatescontinuously at a rate set by an operator.
 2. A pump according to claim1 wherein the control unit compares the pressure detected by thepressure sensor with a target pressure and adjusts the speed of themotor accordingly.
 3. A pump according to claim 1 wherein the controlunit receives a measurement of the current position of the rotor andcompares that position to a desired position of the rotor, and adjuststhe speed of the motor to compensate for any difference between thecurrent position and the desired position.
 4. A pump according to claim1 wherein the control unit determines a desired position of the rotorthroughout its rotary cycle from a lookup table according to the rateset by the operator.
 5. A pump according to claim 4 wherein the desiredposition of the rotor throughout the rotary cycle in the lookup tableentries is determined by the position of the rotor required to maintaina constant output pressure.
 6. A pump according to claim 5 wherein thecontrol unit adjusts the point of entry into the lookup table from whichthe desired position of the rotor is determined according to a targetpressure.
 7. A pump according to claim 6 wherein, during steady-stateoperation of the pump, the target pressure is the pressure detected bythe pressure sensor and subjected to low-pass filtering.
 8. A pumpaccording to claim 4 wherein, if the lookup table does not contain anentry for the output pressure in question, the control unit interpolatesbetween adjacent entries in the lookup table to determine the desiredposition of the rotor throughout the rotary cycle.
 9. A pump accordingto claim 4 wherein the control unit monitors the performance of the pumpover time using the measured pressure and adjusts the entry in thelookup table which is consulted according to the observed performance ofthe pump.
 10. A pump according to claim 1 wherein the motor is a steppermotor.
 11. A pump according to claim 1 further including a sensorarranged to determine the position of the rotor.
 12. A method ofcontrolling a peristaltic pump having a rotor driven by a motor, themethod comprising the steps of: receiving a desired operating rate froman operator; detecting the pressure of the pumped fluid downstream ofthe pump: and adjusting the standard operating speed of the motoraccording to the detected pressure such that the pump operatescontinuously at the desired operating rate.
 13. A method according toclaim 12 further including the steps of: comparing the pressure detectedby the pressure sensor with a target pressure; and adjusting the speedof the motor accordingly.
 14. A method according to claim 12 furtherincluding the steps of: measuring the current position of the rotor;comparing that position to a desired position of the rotor, andadjusting the speed of the motor to compensate for any differencebetween the current position and the desired position.
 15. A methodaccording to claim 12 further including the step of determining adesired position of the rotor throughout its rotary cycle from a lookuptable according to the rate set by the operator.
 16. A method accordingto claim 15 wherein the desired position of the rotor throughout therotary cycle in the lookup table entries is determined by the positionof the rotor required to maintain a constant output pressure.
 17. Amethod according to claim 16 further including the step of adjusting thepoint of entry into the lookup table from which the desired position ofthe rotor is determined according to a target pressure.
 18. A methodaccording to claim 17 wherein, during steady-state operation of thepump, the target pressure is the detected pressure which is thenlow-pass filtered.
 19. A method according to claim 15 further includingthe step of, if the lookup table does not contain an entry for theoutput pressure in question, interpolating between adjacent entries inthe lookup table to determine the desired position of the rotorthroughout the rotary cycle.
 20. A method according to claim 15 furtherincluding the steps of: monitoring the performance of the pump over timeusing the measured pressure; and adjusting the entry into the lookuptable which is consulted according to the observed performance of thepump.